Use const& signature for read-only sub callbacks

[aprotyas]

The constant reference message signatures should be preferred over the
shared pointer to constant message signatures for read-only subscriber
callbacks, as discussed in ros2/rclcpp#1598.

As such, it makes sense to migrate to said signatures in the examples.

Note that this should be backported to Galactic only.

Signed-off-by: Abrar Rahman Protyasha [email protected]

[aprotyas]

CI
Repos file: https://gist.github.com/aprotyas/dca8fd39a17abacc898fa569e6b50fc5
Build args: --packages-up-to examples_rclcpp_minimal_subscriber
Test args: --packages-select examples_rclcpp_minimal_subscriber
Job: https://ci.ros2.org/job/ci_launcher/9790/

• Linux
• Linux-aarch64
• Windows

[wjwwood]

lgtm

This isn't strictly required (there's no problem using ::ConstSharedPtr in this case), but I do agree that const & is easier to read and understand and also has no drawback as a recommendation. So 👍 from me.